The present invention relates to formulations of blended phosphates, particularly those of zinc, and more particularly to an improved preparation of concentrated zinc polyphosphate provided in a stable, alkaline solution having a relatively high pH factor.
Industrial and municipal water supply systems have traditionally been plagued by corrosion problems that adversely affect not only the physical elements of such systems but more importantly the chemical quality of the water. Besides causing deterioration of water mains and other structural members, corrosion in water supply systems can raise the iron concentration of the raw water manifesting so-called "red-water" problems at the point of use and further result in an inability to retain a satisfactory level of chlorine in the water supply due to its loss by chemical reaction with resulting iron oxides. Present water supply systems generally employ continuous monitoring of the water quality and use well-known indicators, such as the Langelier and Ryznar indices, to assess the tendency of the in-line aqueous liquid to be scaleforming or corrosive. By seeking to balance the Langelier saturation index to zero and/or minimize the Ryznar stability index, the characteristic qualities of the supply water that tend to increase its corrosive tendencies, such as low pH factor and decreased alkalinity, can be favorably controlled. Such corrosion-preventive controls within water treatment systems have proven somewhat successful but costly, with the most generally accepted treatment being the controlled addition of corrosion inhibitors.
Some of the most effective corrosion inhibitors being used currently as additives in water systems are blended phosphate-based chemicals including those categorized as orthophosphates and polyphosphates. The orthophosphates are formulations of phosphate salts which are generally produced by the neutralization of phosphoric acid with an alkali metal and which serve to contribute to the process of corrosion inhibition as water softeners and detergent agents. The polyphosphates are complex derivatives of orthophosphates that are converted typically by calcination, to form a chain arrangement of phosphate ions (PO.sub.3.sup.-) held together by P-O-P linkages. These polyphosphates are especially useful in water treatment as effective complexing or sequestering agents that essentially tie-up certain metallic ions in soluble complexes, effectively removing them from the aqueous liquid and preventing their formation of undesirable precipitates or the occurrence of other detrimental side reactions. For example, the complexation of calcium ions using polyphosphates reduces the saturation state of calcium carbonate in the treated water and thereby inhibit scale deposits. Further, the complexation of iron and manganese ions decreases the tendency of water to form stains by inhibiting their oxidation and the resulting precipitation of hydrous oxides of these metals. As a result, polyphosphates are widely used and generally relied upon to clean water mains and reduce "red water" problems that can affect major water supply systems.
The blended phosphate products commonly used for water treatment comprise combinations of the extended chain polyphosphate and orthophosphate chemicals with the intended effect being to provide both the basic corrosion-inhibiting properties of the orthophosphate ions and the sequestering abilities of the polyphosphate. The polyphosphates, however, can break down or hydrolyze over time into a mixture of shorter chain length phosphates and orthophosphate ions. This occurrence, commonly called reversion, is a function of the water chemistry (the pH and metal ions present), its temperature, and the particular type of phosphate chains involved in the water treatment. Since the reversion process adversely affects the overall corrosion-inhibiting properties of the blended phosphate products, improved commercial formulations have developed, particularly employing zinc polyphosphates, that have proven to inhibit the reversion mechanism by controlled addition of metal zinc to the water treatment process. Use of these zinc polyphosphate formulations, however, have been somewhat problematic due to the requirement that they be blended in an acidic environment, generally having a pH factor below 2.0, in order to solubilize the zinc with the polyphosphate ions and reduce undesirable precipitate formations of zinc phosphate or zinc hydroxide. Higher, more effective concentrations of zinc polyphosphate require even more acidic solutions that can actually hasten the reversion process and increase the amount of undesirable precipitates. Of course, the introduction of any acidic solution in the water treatment process will lower the pH factor and reduce the alkalinity of the raw water supply, and, as indicated by both the Langelier and Ryznar formulae, thereby increase the corrosiveness of the water being treated.
While the use of zinc polyphosphates as a corrosion-inhibiting additive remains vital in water treatment systems, their solubilization and continued introduction in acidic solutions can be counter-productive as a corrosion protective treatment. In addition, the strongly acidic solutions heretofore required for effective solubilization of the zinc and polyphosphates ions in high concentrations are very unstable, highly dangerous and further present a variety of safety problems in handling and distribution. A need exists, therefore, for an effective non-acidic preparation of a zinc polyphosphate solution for general use in water treatment systems.